Switch power supplies are currently in the trend of developing towards high frequency and miniaturization, high power density, high efficiency and low cost. Since semiconductor devices function as hard switches and suffer from a great loss, traditional switch circuits fail to improve their own efficiencies and thus become less competitive in the market due to their bulkiness. In view of limitations of the development in the industry of semiconductor devices, the cost, etc., a soft switch circuit topology has become an option for a majority of switch power supply manufacturers to improve competitiveness of products. There are numerous researches and patents on soft switch circuits, and one of them is a soft switch circuit of Auxiliary Resonant Commutated Pole (ARCP) in the form of “inductor plus switch” in series, which has won the popular favor of those skilled in the art of switch power supplies due to its simple hardware circuit, easiness to control and satisfactory effect. Chinese Utility Model Patent ZL 200620131113.6, for example, discloses an ARCP soft switch circuit, which is an improvement on such a soft switch circuit.
The operation principle of a traditional hard switch circuit is as follows: as illustrated in FIG. 1, positive and negative direct current power supplies ±½Ud and main power switch devices SW1 and SW2 constitute a main power half bridge inverter circuit. The main power switch devices SW1 and SW2 are controlled to be turned on and turned off constantly to generate a high frequency pulse voltage of ±½Ud at the point B. A power frequency output voltage required according to the design across a resonant capacitor C3 and a main power filter current I1 across a inductor L1 are generated through the inductor L1 and the resonant capacitor C3 of a main power filter circuit. Here, the main power switch devices SW1 and SW2 belong to traditional hard switches, which suffer from a great loss.
In order to lower the loss of the main power switch devices SW1 and SW2, two unidirectional auxiliary switch devices SW3 and SW4 and a resonant inductor L2 are added in the ARCP soft switch circuit, and also resonant capacitors C1 and C2 with larger capacitances relative to parasitic capacitances of SW1 and SW2 are arranged in parallel across the main power switch devices SW1 and SW2 respectively. The auxiliary switch devices SW3 and SW4 are controlled to be turned on and turned off to generate a resonant current I2 across the resonant inductor L2 in the same direction as the main power filter current I1, and the resonance of the resonant inductor L2 and the resonant capacitor C3 achieves the zero-voltage turn-on of the main power switch devices SW1 and SW2. Also the parallel arrangement of the resonant capacitors C1 and C2 with larger capacitances relative to the parasitic capacitances of the main power switch devices SW1 and SW2 across SW1 and SW2 respectively achieves zero-voltage turn-off of the main power switch devices. Thus, the ARCP soft switch circuit can achieve both zero-voltage turn-on and zero-voltage turn-off of the main power switch devices SW1 and SW2 to thereby significantly lower the loss of the main power devices. Further regarding the newly added auxiliary switch devices SW3 and SW4, no abrupt change of the current will occur due to the presence of the resonant inductor L2 in series therewith to thereby achieve zero-current turn-on, and a reasonable control on the times to turn on and turn off auxiliary switch devices SW3 and SW4 can achieve zero-current turn-off of the auxiliary switch devices SW3 and SW4, so that the newly added auxiliary switch devices SW3 and SW4 can operate in a state of both zero-current turn-on and zero-current turn-off with a very small switching loss.
As can be apparent from the foregoing analysis, the ARCP soft switch circuit achieves both zero-voltage switching of the main power switch devices SW1 and SW2 with a reduced loss and zero-current switching of the auxiliary switch devices SW3 and SW4 with a very small switching loss that can substantially be negligible. Thereby, a significantly improved overall operation efficiency of the circuit, a greatly lowered overall loss and an remarkably reduced volume of the entire equipment are achieved, and hence product competitiveness is improved.
Although the ARCP soft switch circuit has the foregoing advantages of low loss and high efficiency, this circuit topology has been identified in practice with an apparent drawback, i.e., a freewheeling diode D4 can not be normally turned on in the positive half cycle when the main power filter current I1 is small, particularly around a zero crossing point, which may cause the following adverse consequences:
1) A large impact current across the resonant capacitors C1 and C2 may cause shorten lifetime and even failure thereof.
2) A large impact current across the main power switch device SW2 may cause lowered reliability and even failure thereof.
3) A high spike voltage across the main power switch device SW1 may cause breakdown and consequential failure thereof.
All of the foregoing three adverse consequences may directly cause abnormal operation of the circuit and even impossible normal operation of the entire equipment. Similarly, a freewheeling diode D3 being not normally turned on in the negative half cycle may cause the following adverse consequences:
1) A large impact current across the resonant capacitors C1 and C2 may cause shorten lifetime and even failure thereof.
2) A large impact current across the main power switch device SW1 may cause lowered reliability and even failure thereof.
3) A high spike voltage across the main power switch device SW2 may cause breakdown and consequential failure thereof.